Multilayer ceramic capacitors generally have alternating layers of ceramic dielectric material and conductive electrodes. Various types of dielectric materials can be used and various types of physical configurations have been used. Capacitors for high voltage performance have been produced for many years using a “series design”. In the series design the charge is stored between the floating electrode and electrodes connected to the terminals on either side as shown for a single floating electrode designs in FIG. 1. This compares to a standard capacitor design shown in FIG. 2 in which the electrodes alternatively connect to different terminals and the charge is stored between these electrodes. The capacitance for these designs is given by:C=orAN/T     Where C=Capacitance in F    o=Permittivity of Free Space=8.854×10−12Fm−1     r=Permittivity of the Ceramic Material, a material dependent dimensionless constant    A=Effective Overlap Area of Electrodes m2     N=Number of electrodes−1    T=Fired Active Thickness of Ceramic Separating the Layers
However, in the case of the series design the effective overlap area is significantly reduced. The advantage of the series design is that the internal voltage acting on the electrodes is halved for the single floating electrode. It is possible to further separate the floating electrode to give more than one floating electrode per layer to reduce the internal voltage but this also lowers the effective overlap area reducing capacitance. The average voltage breakdowns (n=50) for 27 lots of case size 1812 MLCCs, 47nF±10% standard designs and the same number of case size 1812, 22nF±10% single floating electrode series designs are shown in FIG. 3. In all these cases the fired active thickness separating the electrodes was 0.0023″, 58 microns with an overall thickness of 0.051±0.003″ (1.30±0.08 mm) for the standard design and 0.068±0.003″ (1.73±0.08 mm) for the series capacitors. The length and width dimensions were 0.177±0.010″ (4.50±0.25 mm) and 0.126±0.008″ (3.20±0.20 mm) respectively for all these 1812 case size capacitors. Cross-sections of the 1812 standard design and the single electrode series design are shown in FIGS. 4 and 5 respectively.
In addition to the internal voltage withstanding capability of these MLCCs it is also critical that these parts are resistant to arc-over from the capacitor terminals. U.S. Pat. No. 4,731,697, to McLarney discloses a surface electrode with portions of the margin covered by a further dielectric layer to prevent arc over that requires laser trimming. However, it is important to note that exposed electrodes are subject to corrosion. Also the properties of exposed electrodes are significantly impacted by the environment factors, such as humidity, limiting the applications in which these capacitors can be used.
U.S. Pat. No. 6,627,509 to Duva discloses a method for producing surface flashover resistant capacitors by applying a para-poly-xylylene coating to the surface of multilayer ceramic capacitors followed by trimming the excess material from the terminals. In this case significant costs are associated with coating of the capacitors. Furthermore, the coating may not be compatible with the circuit board assembly processes and the presence of organic coatings in some electronic application such as satellites is limited because of out gassing concerns.
Thus, despite various efforts to reduce produce capacitors with high voltage breakdown and which minimize occurrence of arc over, problems remain. What is needed is an improved high voltage capacitor.